Deletion of FoxO1 leads to shortening of QRS by increasing Na⁺ channel activity through enhanced expression of both cardiac Na_V1.5 and β3 subunit

Our in vitro studies revealed that a transcription factor, Forkhead box protein O1 (FoxO1), negatively regulates the expression of Na_V1.5, a main α subunit of the cardiac Na⁺ channel, by altering the promoter activity of SCN5a in HL-1 cardiomyocytes. The in vivo role of FoxO1 in the regulation of cardiac Na_V1.5 expression remains unknown. The present study aimed to define the role of FoxO1 in the regulation of Na_V1.5 expression and cardiac Na⁺ channel activity in mouse ventricular cardiomyocytes and assess the cardiac electrophysiological phenotype of mice with cardiac FoxO1 deletion. Tamoxifen-induced and cardiac-specific FoxO1 deletion was confirmed by polymerase chain reaction (PCR). Cardiac FoxO1 deletion failed to result in either cardiac functional changes or hypertrophy as assessed by echocardiography and individual ventricular cell capacitances, respectively. Western blotting showed that FoxO1 was significantly decreased while Na_V1.5 protein level was significantly increased in mouse hearts with FoxO1 deletion. Reverse transcription-PCR (RT-PCR) revealed that FoxO1 deletion led to an increase in Na_V1.5 and Na⁺ channel subunit β3 mRNA, but not β1, 2, and 4, or connexin 43. Whole patch-clamp recordings demonstrated that cardiac Na⁺ currents were significantly augmented by FoxO1 deletion without affecting the steady-state activation and inactivation, leading to accelerated depolarization of action potentials in mouse ventricular cardiomyocytes. Electrocardiogram recordings showed that the QRS complex was significantly shortened and the P wave amplitude was significantly increased in conscious and unrestrained mice with cardiac FoxO1 deletion. Na_V1.5 expression was decreased in the peri-infarct (border-zone) of mice with myocardial infarction and FoxO1 accumulated in the cardiomyocyte nuclei of chronic ischemic human hearts. Our findings indicate that FoxO1 plays an important role in the regulation of Na_V1.5 and β3 subunit expressions as well as Na⁺ channel activity in the heart and that FoxO1 is involved in the modulation of Na_V1.5 expression in ischemic heart disease.